Fukushima and Chernobyl: What makes a level 7 nuclear crisis?
The crisis at the Fukushima nuclear power plant in Japan is now being rated at the same level as the Chernobyl disaster 25 years ago.
Japanese officials say raising its severity to level 7 (the highest) reflects the total amount of radiation released since the emergency began, rather than any sudden incident.
Fergus Walsh explains.
- 12 April 2011
- BBC News